Four bars are joined together with hinges at their ends to form a rhomb (yellow bars). One hinge of this rhomb is connected to one of the stationary shafts (green) of a four-bar linkage (FBL) with a common axis. The second hinge of the rhomb is connected to a moving hinge of the FBL (red bar), and the third hinge of the rhomb is connected to the other moving hinge of the FBL (with a red bar). The third and fourth hinges of the rhomb are connected to a foot (three lavender bars). With suitable selection of the bar lengths, the closed curve followed by the foot is similar to an airfoil; that is, it is close to flat at the bottom and arched above. This feature of the movement can be used to make walking machines, such as those made by the artist-engineer Theo Jansen.